• H test for a single population
• 1- and 2-sided tests
• H test for two populations
• Assumptions of parametric tests

What we will cover today:

• Assumptions of parametric tests and how to run them
• Statistical vs. biological significance - is there a difference
• What to do when assumptions fail
  • Robust tests
  • Rank-based tests
  • Permutation tests

Lets work with the Lake Trout data as the weights are pretty cool in this one and will bakc up the main points of this lecture.

This is easily translated into the mice weight data from Vancouver or the pine needle data and we could do those too on the fly if you want….

lake trout

• T-tests are parametric tests

  • Parametric tests: specify/assume probability distribution from which parameters came

• Non-parametric tests: no assumption about probability distribution

• Mukasa et al 2021 DOI: 10.4236/ojbm.2021.93081

• If assumptions of parametric test violated, test becomes unreliable
• This is because test statistic may no longer follow distribution
• Most parametric tests robust to mild/moderate violations of below assumptions

• Basic assumptions of parametric t-tests:
• Normality, equal variance, random sampling, no outliers
• Normality: Samples from normally distributed population
  • Graphical tests: histograms, dotplots, boxplots, qq-plots
  • “Formal” tests: Shapiro-Wilk test - sometimes not useful

• Equal variance: samples are from populations with similar degree of variability
  • Graphical tests: boxplots
  • “Formal” tests: F-ratio test
• Parametric tests most robust to violations of normality and equal var. assumptions when samples sizes equal

• Normality, equal variance, random sampling, no outliers
• Random sampling:
  • samples are randomly collected from populations; part of experimental design
• Necessary for sample -> population inference

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• Normality, equal variance, random sampling, no outliers
• No outliers: no “extreme” values that are very different from rest of sample
  • Graphical tests: boxplots, histograms
  • “Formal tests”: Grubb’s test - no one really does this
  • Note: outliers also problem for non-parametric tests

• Statistical significance: difference unlikely due to chance
• Says nothing about biological significance of difference!
• With large sample size can detect very small differences between populations
• E.g.: consider 2 lake trout populations are the lengths the same
  • Island Lake and NE 12
    • Ho: µ~size A~ = µ~size B~
    • Ha: µ~size A~ ≠ µ~size B~

• Size of A: 5.05 (± 2.00 SD)mm, size of B: 5.00 (± 2.00 SD)mm
• Sample 50, 200, 30,000 individuals from each pop:
  • n = 50: t = 0.32, df = 98, p-value = 0.75
  • n = 200: t = 0.058, df = 398, p-value = 0.95
  • n = 30,000: t = -4.47, df = 59998, p-value = 7.996*10^-6

• Finally, statistically significant difference…
  • Meaningful?
  • Ecologically significant?
  • Statistics can’t answer this question
• IMPORTANT to report info that can assess biological significance
• “A two-tailed, two-sample independent t-test showed significant difference in size between pop. A (4.99 mm ± 1.99 SD) and pop. B (5.06 mm ± 1.99 SD) at á=0.05 (t = -4.47, df = 59998, p-value < 0.0001).”

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• Basic assumptions of parametric t-tests:
• Normality, equal variance, random sampling, no outliers
• What to do if assumptions are violated?

• t-tests have several assumptions.
• Alternative tests, with more relaxed assumptions, are available to statisticians.
• In which case would you use the following tests?
  • Welch’s t-test: when distribution normal but variance unequal
  • Permutation test for two samples: when distribution not normal (but both groups should still have similar distributions and ~equal variance)
  • Mann-Whitney-Wilcoxon test: when distribution not normal and/or outliers are present (but both groups should still have similar distributions and ~equal variance)

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ne12_histo_plot+ne12_box_plot + ne12_qq_plot + plot_layout(ncol=1)

• QQ-plots: tool for assessing normality
  • On x- theoretical quantiles from SND
  • On y- ordered sample values
  • Deviation from normal can be detected as deviation from straight line

• In some cases, data can be mathematically “transformed” to meet assumptions of parametric tests
• this can be done in r and usually involves
  • log10 transformations
  • square root transformations
  • and many others… I will have a description soon

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source

• Welch’s t-test
  • common “robust” test for means of two populations
  • Robust to violation of equal variance assumption, deals better with unequal sample size
  • Parametric test (assumes normal distribution)
  • Calculates a t statistic but recalculates df based on samples sizes and s

• Welch’s t-test
  • t.test(y1, y2, var.equal = FALSE, paired = FALSE)
  • will use the Welch approach
• T-test
  

# T test for lenght
# Perform standard t-test
t_test_length_result <- t.test(
  length_mm ~ lake, 
  data = isl_ne12_df,
  var.equal = TRUE  # Standard t-test with equal variance assumption
)

# Perform Welch's t-test (unequal variances)
welch_test_length_result <- t.test(
  length_mm ~ lake, 
  data = isl_ne12_df,
  var.equal = FALSE  # Welch's t-test
)

• Rank-based tests: no assumptions about distribution (non-parametric)
• Ranks of data: observations assigned ranks, sums (and signs for paired tests) of ranks for groups compared
• Mann-Whitney U test common alternative to independent samples t-test
• Wilcoxon signed-rank test is alternative to paired t-test

# Perform Mann-Whitney U test (Wilcoxon rank-sum test)
mann_whitney_length_test <- wilcox.test(
  length_mm ~ lake, 
  data = isl_ne12_df,
  exact = FALSE,  # Use approximation with ties
  conf.int = TRUE  # Calculate confidence interval
)

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[1] "Mann-Whitney U test results length:"

    Wilcoxon rank sum test with continuity correction

data:  length_mm by lake
W = 3226, p-value = 7.814e-08
alternative hypothesis: true location shift is not equal to 0
95 percent confidence interval:
 262.0000 426.9999
sample estimates:
difference in location 
                   357 

• Assumptions: similar distributions for groups, equal variance
• Less power than parametric tests
• Best when normality assumption can not be met by transformation (weird distribution) or large outliers

A: n= 15, y= 8, s= 4 B : n= 15, y= 10, s= 5

Approach A vs. B

T-test df= 28 t= -3.53 p= 0.0014 M-W U (Wilcoxon’s) W= 41 p= 0.002

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[1] "Standard t-test results for lenght_mm:"

    Two Sample t-test

data:  length_mm by lake
t = 8.616, df = 331, p-value = 2.888e-16
alternative hypothesis: true difference in means between group Island Lake and group NE 12 is not equal to 0
95 percent confidence interval:
 270.1939 430.0761
sample estimates:
mean in group Island Lake       mean in group NE 12 
                  698.200                   348.065 

[1] "Mann-Whitney U test results length:"

    Wilcoxon rank sum test with continuity correction

data:  length_mm by lake
W = 3226, p-value = 7.814e-08
alternative hypothesis: true location shift is not equal to 0
95 percent confidence interval:
 262.0000 426.9999
sample estimates:
difference in location 
                   357 

• Ho: µ_A = µ_B, Ha: µ_A ≠µ_B
• Calculates the differnce ∆ in means between two groups

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• Randomly reshuffle observations between groups (keeping n_NE 12=323 and n_Island=10), calculate ∆
• Repeat >1,000 times
• Record proportion of the different means i
• This is equivalent to p-value and can be used in “traditional” H test framework
• For a graphical explanation:
  • Graphical Explanation